Since it is difficult for the UWB communication, which occupies frequencies of a ultra wide bandwidth, to maintain a dedicated band, it shares a frequency band with existing communication systems. Accordingly, it is feared that “interference” can occur between the UWB radio communication apparatus and the existing communication systems, resulting in jamming on transmission signals to each other.
As for the interference the UWB radio communication apparatus gives to the existing communication systems, a technique called DAA (Detect and Avoid), which circumvents the interference by detecting the presence or absence of a signal from the existing communication systems and by putting a notch in the UWB transmission wave at a particular frequency at which the signal occurs, has already been put to practical use.
For example, Patent Document 1 discloses a UWB radio communication apparatus employing an MB-OFDM (MultiBand-Orthogonal Frequency Division Multiplexing) communication scheme capable of implementing the detection in the DAA appropriately by preventing a dead zone from occurring near its local frequency by employing a Low-IF technique as a downconverter on the receiver side.
On the other hand, as for the interference the UWB radio communication apparatus receives from the existing communication system, the existing communication systems do not have duty to avoid the interference. Accordingly, when the UWB radio communication apparatus detects interference, a higher rank MAC (Media Access Control) layer usually avoids using the band undergoing the interference by deciding and making band switching.
However, there is a possibility that a similar interference can occur in the band to which the switching is made. In addition, when using another band is limited by law, the interference avoidance by band switching cannot be achieved.
FIG. 5 is a diagram for explaining interference the UWB radio communication apparatus receives from an existing communication system. Jamming as shown in FIG. 5 occurs because of a transmission wave from an existing communication system that shares a band with the UWB radio communication apparatus or because the out-of-band spurious radiation from an existing communication system using an adjacent band enters the band of the UWB radio communication. Here, as compared with the bandwidth of the UWB, 528 MHz, that of the existing communication systems is narrow of about several tens of MHz. In addition, since the UWB scatters the power to a wide band, the power density is much smaller than that of other communication systems.
The signal of the foregoing band, which is received with an antenna and is amplified, undergoes frequency conversion to a baseband signal by being multiplied by a local signal LO with the same frequency as the reception center frequency frf. After that, its low-frequency component, which passes through a low-pass filter (LPF) with passing characteristics indicated by broken lines in FIG. 5, is amplified and demodulated through a variable gain amplifier (VGA). In this case, the variable gain amplifier (VGA) carries out AGC (Automatic Gain Control) in such a manner that the input signal level to the AD converter becomes constant.
As is clearly seen from the baseband signal after passing through the LPF shown in FIG. 5, the jamming entering the band cannot be attenuated even if it is passed through the low-pass filter (LPF) or a band-pass filter (BPF).
Accordingly, when both the UWB desired wave and jamming are present within the band, the gain is controlled with respect to the signal level including the jamming.
Accordingly, depending on the level difference between the jamming and the desired wave (D/U ratio), enough amplitude (resolution of the AD conversion) for receiving the desired wave signal cannot be obtained, offering a problem of causing a UWB frame detection error or a receiving error such as a bit error.
The problem cannot be solved by the invention disclosed in the Patent Document 1 that proposes the technique of avoiding the interference the transmitter provides to the existing communication systems.
The present invention is implemented to solve the foregoing problem. Therefore it is an object of the present invention to provide a radio communication apparatus capable of avoiding interference due to jamming from an existing communication system by attenuating the jamming, which has passed through the conversion to the baseband signal frequency, by shifting it to the outside of the pass band of the low-pass filter in the UWB communication based on an MB-OFDM communication scheme.